With increasing communication traffic in a backbone network system, a demand for an optical communication system that can transmit signals over long distances and that can transmit a large amount of data increases. In order to increase an amount of data transmitted, the optical communication system generally employs a digital coherent transmission scheme in which a transmitter and a receiver compensate for signal degradation occurring in a transmission line using digital signal processing.
In wavelength division multiplex (WDM) optical communication performed in optical communication systems, a problem of degradation of an optical signal caused by a non-linear optical effect in an optical transmission line (an optical fiber) arises. In a long-distance WDM optical communication, one of reasons for signal degradation is cross phase modulation (XPM), which is one of non-linear optical effects. In particular, in a so-called super channel, which is a scheme in which a channel includes a plurality of sub-carriers, a distance between the wavelengths of adjacent carriers is small. Accordingly, the degradation caused by XPM is significant.
A known compensation scheme for XPM occurring in a transmission line over a plurality of spans has yet not been developed in the pre-equalization technology.
Refer to, for example, Japanese Laid-open Patent Publication No. 11-149064 and Zhenning Tao, et al. “Simple Fiber Model for Determination of XPM Effects”, Journal of Lightwave Technology, vol. 29, No. 7, Apr. 1, 2011.
In some applications of an optical communication system, a plurality of repeaters is disposed in the optical transmission line extending between a transmitting apparatus and a receiving apparatus. Part of the optical transmission line located between two adjacent repeaters is referred to as a “span”.
To reduce signal degradation caused by XPM, the pre-equalization technology has been developed. In the pre-equalization technology, a phase change that is opposite to the phase change caused by XPM occurring in a transmission line is given to a signal on the transmitter side in advance. As an existing technology for the wavelength division multiplexing optical communication, a technology that pre-equalizes XPM occurring in a first span has been developed. However, this technology does not take into account pre-equalization of XPM occurring in the second and subsequent spans. Accordingly, the demand for pre-equalization of XPM occurring in the second and subsequent spans is increasing. In order to pre-equalize XPM, it is desirable that the data waveform information regarding signals of all of the channels be shared by transmitters for all of the channels (i.e., for all of the wavelengths). However, if the data waveform information regarding signals of all of the channels is shared, the data capacity increases and, therefore, it is not practical. In addition, in polarization and wavelength division multiplexing optical communication, it is desirable that signal degradation caused by polarization crosstalk be taken into account.